It would be interesting to see approximately at what time flow separates from the wedge wall giving rise to the vortex phenomena mentioned above. It is well known that whenever vorticity changes sign along a solid wall, flow separation takes place paving the way for the creation of a vortex ([1]}, [2]}, [3]}). As such, the number of vortices will depend upon the total number of changes of sign in vorticity values. We illustrate this phenomenon in figures REF (a)-(f). In figures REF (a), (c) and (e), we have plotted the vorticity distribution against the \(x\) -coordinate of the wedge wall and different times, while figures REF (b), (f) and (f) depicts the corresponding combined streamfunction and vorticity contours. In figure REF (a), one can clearly see that at time \(t=0.02\) , there is no change of sign of vorticity along the wall, similar to the situation depicted in figure REF (e) at time \(t=0.01\) . On the other hand, at a later time \(t=0.06\) , one can see a very small zone of negative vorticity value which corresponds to the white region in figure REF (b), enclosed by the (primary) vortex bounded by the \(\psi =0\)  streamline. This clearly demonstrates that the first flow separation takes place in the interval \(t \in (0.02,0.06]\) . Likewise, in figure REF (c), one can see that at \(t=0.20\) , there is only one negative vorticity zone across the wedge wall. However, at \(t=0.25\) , one can see a very narrow zone of positive vorticity inside the primary vortex zone (see the enlarged view in the inset), giving rise to the second separation. This secondary vortex can be seen more clearly in figure REF (d), where the region in green colour is enclosed by the \(\psi =0\)  streamline and the primary vortex that engulfs the secondary vortex is represented by the white region (see also figures REF (g) and REF (g) at time \(t=0.30\) ). This indicates a second flow separation in the interval \(t \in (0.20,0.25]\) . Meanwhile, the secondary vortex grows in size and strength and at around \(t=0.6\)  divides the primary vortex into two chambers, one of which is the core and the other one gives rise to a tertiary recirculating zone. It becomes more prominent at \(t=0.90\)  onwards, which can also be observed in figures REF (d) and (h). The vorticity distribution along the wall in figure REF (c) at \(t=0.90\)  clearly demonstrates these three primary, secondary and tertiary recirculation zones, with figure REF (f) showing the small negative vorticity region in the neighbourhood of the wedge-tip.
